1. Field of the Invention
The present invention relates to a DME (Distance Measuring Equipment) system, and specifically, it relates to a GBAS (Ground Based Augmentation System) which broadcasts a reinforcing data of the GPS in the DGPS from a ground unit.
2. Description of the Prior Art
Previously, an amplitude of the pulse transmitted from a DME ground unit has automatically been controlled so as to become a constant value by a method shown in Japanese Patent Publication No. 2-47710. Accordingly, there is no data broadcasting function in pulses other than that of the Morse code which is transmitted once for every 30 seconds.
On the other hand, in case of constructing a landing guidance system using the GPS, it is necessary to transmit the data to reinforce the accuracy and integrity of the GPS. Therefore, at present, a data broadcasting system using a VHF which is used by an ILS (Instrument Landing System) and a VOR (VHS Omnidirectional Radiorange) is being developed.
However, this frequency band has already been assigned to a lot of facilities, and the interference with the ILS, the VOR and the aeronautical radio is inevitable. For obtaining a sufficient number of stations, there is such a problem that it is necessary to wait for the removal or service stop of the ILS or the VOR.
A conventional DME system gives attention to the pulse width, and works so as to keep the sliced pulse width constant, and as a result, the amplitude is also kept constant, and therefore, it is impossible to perform the data broadcasting by the amplitude modulation method using this DME unit.
Furthermore, as mentioned above, the data broadcasting system using the VHF has such a disadvantage that the interference with the ILS or the VOR is inevitable in this country where the assignment of frequencies has already been saturated. In order to solve this problem, there is a method of improving the spectrum, but it has a disadvantage of requiring a large-scale unit.
It is an object of the present invention to provide means by effectively utilizing the frequency resource of the DME/TACAN band specified for the aircraft band, and which overlaps the GPS augmentation data onto the distance information and broadcasts that.
It is another object of the present invention to achieve the GBAS function without mounting any new unit on the aircraft side.
A DME system of the present invention is characterized by modulating the amplitude of the transmitting pulse of a DME ground station within an allowable range so as to transmit information other than the distance information and the station identification code (ID).
In the present invention, in order to clearly express the start point of the data and to make the bit synchronization easy, a training sequence and a unique word which include xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d and have previously been determined are added just before the data by header adding means. In this header, the training sequence is used as the reference value of level xe2x80x9c1xe2x80x9d and level xe2x80x9c0xe2x80x9d showing the data, and the unique word is used for preventing a wrong detection of a signal because of wave form degradation caused by noise or propagation.
The present invention comprises: a ground unit having level modulating means which modulates the output of a conventional DME transmitter in a way of level modulation for each paired pulse stimulation according to a signal including xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d with a header; a detecting means which detects level xe2x80x9c1xe2x80x9d and level xe2x80x9c0xe2x80x9d of the video output of a conventional airborne DME receiver; and an air borne DME unit having a decoding means which judges xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d from this level signal and the receiving pulse timing.
The header adding means adds a header including a previously determined code string of xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d just before the inputted data. This header includes a training sequence and a subsequent unique word. The training sequence includes xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d which are alternately repeated, and clearly expresses the level average of xe2x80x9c1xe2x80x9d and xe2x80x9c0.xe2x80x9d Furthermore, the unique word shows the start of information.
The level modulating means uses a method of changing the peak level of each paired pulse stimulation according to the information of xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d for each paired pulse stimulation by an attenuator or a pulse modulating means, and it changes the level of the output of a conventional ground DME transmitter according to this information of xe2x80x9c1xe2x80x9d and xe2x80x9c0.xe2x80x9d At that moment, the transmitter separates a pulse showing xe2x80x9c1xe2x80x9d and a pulse showing xe2x80x9c0,xe2x80x9d and keeps the pulse width constant, and it maintains the spectrum within the prescribed value.
The level detecting means provided in the airborne DME unit determines the average value of the receiving pulse level from the training sequence pulse received for a certain time, and it makes this average level (mean level) the threshold value to judge that a pulse higher than that value is xe2x80x9c1xe2x80x9d and a pulse lower than that value is xe2x80x9c0.xe2x80x9d
The decoding means generates a signal string of xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d from this judgment value and the timing of the pulse receiving, and in the meantime, it detects a unique word from the signal string by correlation processing, and separates and outputs the data.
According to the present invention, for example, a header is added to the DGPS (Differential GPS) augmentation reinforcing data, and the transmitting pulse level of a conventional ground DME system is modulated, and the data is broadcast to an airborne system. In the airborne system, a threshold value of xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d is generated by the level detecting means, and the start point of the data is detected by the header detecting means, and the DGPS augmentation data is supplied to the airborne system. Consequently, it is possible to achieve the function of broadcasting a data by overlapping the data onto the distance information of the DME. Furthermore, since the DME uses the L band, the radio interference with the existing ILS, VOR, aircraft radio transmission, and broadcasting station can be avoided.